1. Field of the Invention:
This invention relates to an improved method and apparatus for recovery of a reduced, purified metal. More specifically, the invention relates to an improved reduction, distillation furnace for recovery of a reduced metal and any unused reducing metal.
2. Description of the Prior Art:
Technical and patent literature reports numerous methods for separating and recovering various metals. Frequently the processes will involve the isolation of a purified salt of the metal to be recovered and a subsequent chemical reduction involving the addition of at least a stoichiometric amount of a more active reducing metal. The reduction step will then be followed by an appropriate separation of the desired elemental form of the metal from any excess reducing metal and reducing metal salt formed during the displacement reaction. In contemporary commercial scale production (i.e., production involving tonnage quantities of free metal) and in particular in the recovery of such metals as titanium, zirconium, hafnium, and the like, the overall reduction and recovery process is a batchwise series of high temperature and high vacuum steps.
Thus, for example, in the production of zirconium intended for use as a structural material in nuclear reactor applications, a hafnium-free, highly purified zirconium halide (e.g. ZrCl.sub.4) is loaded into a vessel or retort. Typically, this reduction retort is a cylindrical soft iron vessel having a centrally located cylindrical (angel-food cake pan) inner wall with the ZrCl.sub.4 loaded into the doughnut shaped annular cavity. After isolation of the ZrCl.sub.4 in this annular cavity and cooling of the retort, it is placed on top of a crucible filled with at least a stoichiometric amount of a reducing metal (e.g. magnesium, sodium or the like). A lid is sealed on top of the retort and the crucible is welded to the retort thus creating a hermetically sealed retort assembly. This retort assembly is then evacuated to create the desired oxygen-nitrogen free inert atmosphere and placed in a reduction furnace wherein the retort assembly is heated to a temperature of the order of approximately 825.degree. C. for a sustained period of time. Under these conditions the ZrCl.sub.4 will evaporate and diffuse into the lower crucible containing the magnesium reducing metal and therein react, producing elemental zirconium and magnesium chloride.
Upon completion of the reduction reaction, the retort assembly is cooled and the crucible and retort are milled apart. The retort is then cleaned and returned to the ZrCl.sub.4 charging step. The crucible containing the reaction products is sent to an area where the products are removed and most of the MgCl.sub.2 layer is physically separated from the sponge zirconium layer. A series of zirconium sponge layers is collected from several crucibles (reduction batch runs) and stacked within the upper portion of a distillation vessel, typically on a support grid separating the upper and lower compartments. This distillation vessel is then hermetically sealed and under vacuum (&lt;20 microns) the upper compartment is maintained at a high temperature (approximately 1050.degree. C.) for a period sufficient to melt and distill the MgCl.sub.2 and any unreacted magnesium. After cooling and conditioning the distillation vessel, the magnesium chloride and magnesium in the lower compartment is removed and the desired zirconium sponge disc is withdrawn from the support grid and sent on to final product handling and fabrication.
There are several shortcomings associated with such a process. First it is labor intensive in materials handling and requires two full heat-ups from room temperature. Also, the process is long and time consuming. The in to out process time cycle on a commercial scale is approximately 16 to 20 days in length.